This invention relates to a nonlinear resistor comprising a sintered body containing zinc oxide as its principal component in combination with additives such as bismuth oxide and boron oxide, and a method for producing such a resistor.
Nonlinear resistors comprising molded and sintered bodies of zinc oxide with additives such as bismuth oxide, manganese oxide, cobalt oxide, antimony oxide, chromium oxide, boron oxide and the like are widely used for voltage stabilizers, surge absorbers, arresters, etc. These nonlinear resistors are excellent in non-linearity of voltage-current characteristics in comparison with the nonlinear resistors made of silicon carbide, but they involved problems in that their properties are subject to deterioration after surge absorption or longtime application of rated voltage, causing a gradual increase of leakage current and finally inducing thermal runaway. As to the property deterioration, it was known the following facts: (1) when a nonlinear resistor element is heated in a nitrogen gas atmosphere, there occurs the same pattern of property deterioration as that caused by voltage application, and (2) the element which suffered the property deterioration can recoup its original properties when the element is heat treated in air. Taking these facts into consideration, causes of the property deterioration seems to be that oxygen in the crystal grain bondary layers in the sintered body or oxygen adsorbed on the grain surfaces is released into the ambient atmosphere at the time of voltage application, resulting in a lowered potential barrier in the grain boundary layers to increase a leakage current.
The following methods have been proposed for minimizing such property deterioration of the zinc oxide based nonlinear resistors by improving stability to voltage application:
(1) Bismuth oxide is diffused from the entire surface of the sintered body (e.g., U.S. Pat. No. 3,723,175). PA1 (2) The firing temperature for the sintered body or the temperature of the heat treatment after firing is controlled to elevate the ratio of .gamma.-Bi.sub.2 O.sub.3 phase in the Bi.sub.2 O.sub.3 phase (e.g., U.S. Pat. Nos. 4,046,847, 4,042,535 and 4,165,351). PA1 (3) Boron oxide or glass containing boron oxide is added (e.g., U.S. Pat. No. 3,663,458).
However, even the zinc oxide based nonlinear resistors incorporating said techniques were still unsatisfactory in that they could not maintain stabilized properties in all possible use conditions or that they were found defective in certain properties, particularly in long-duration current impulse withstand capability. The term "long-duration current impulse" used here refers to a surge with a pulse width of 2 msec and is supposed to simulate a switching surge.